1. Field of Invention
The present invention relates generally to composite laminated structures, in particular those containing angled ply orientations to achieve desirable improved physical properties, together with methods of manufacturing and using such structures.
2. Description of Related Art
Conventional composite laminated structures are generally designed to emulate the strength characteristics of conventional metal-based laminate materials and as such are constrained to designs having layers of plies that are both symmetrical and balanced. Such conventional structures when so constrained and containing at least three ply layers formed from black carbon fibers, are commonly referred to in the art as “black aluminum” due to their combined carbon makeup and metal-emulating characteristics.
Symmetric laminates involve a reflective or mirror-image equivalence of ply orientation about their mid-plane, while balanced laminates involve an equal number of positively (+) and negatively (−) oriented plies across their entirety. Such constraints have traditionally remained unchallenged due to concerns that conventional composite laminated structures will undesirably warp upon cool down from a curing temperature or increased residual stress when the operating temperature changes.
Symmetric laminates have been traditionally formed by stacking the multiple layers of various unidirectional and woven fabric plies in such a manner that the composite laminate exhibits a mirror-image of itself about a mid-plane of the structure. Such lamination processes are generally time and labor intensive as well as being prone to error, requiring precision ordering of the respective composite layers and may result in an unnecessary number of plies, which may contribute to excessive process waste and cost. Still further symmetric laminates have historically proven cumbersome when seeking to taper the exterior surface of a structure, due at least in part to the desire to maintain symmetry throughout, even when dropping ply layers to form the taper. In addition, as the individual or a pair of symmetric plies with substantially the same orientation is dropped to form a taper, the laminate stacking sequence and thus the material's strength characteristics, are altered.
Balanced laminates, like symmetric ones described above, have been traditionally formed by stacking multiple layers of various unidirectional plies at a plurality of precise orientations with relatively large angles between them. For example, each off-axis ply, such as a +45° ply is typically matched and mirrored by a −45° ply. In addition, a common practice was to have four-ply orientations incorporating angles of −45°, 0°, +45°, and 90°. Alternative, three-ply orientations were also common, such as 0°, ±45° configurations. Critical was that the number of positive (+) and negative (−) oriented plies remain equal.
Balanced and symmetric laminates of this nature have historically created difficulty when trying to minimize laminate thickness, requiring ever thinner plies as the only option. Tapering complexities have existed in these structures as well, given that dropping of particular plies or groups thereof must not disturb the desired symmetry and balance. Further, balanced laminates are orthotropic, where deflection and rotation resulting from bending and twisting moments are uncoupled. This structural response is analogous to that of isotropic materials like metal.
Although not customary in the art, coupled bending and twisting moments may provide desirable deformation characteristics, in particular, permitting designers to reliably predict bending from twisting and cause the two to work against each other, leading to a reduced degree of deflection and/or rotation not possible with orthotropic and isotropic materials. This can be advantageous for long and thin structures, such as for example wind turbine blades, helicopter rotor blades, aircraft wings and tails, and the like, where tip deflection can be reduced in one direction by use of this bend-twist coupling of an unbalanced laminate, but can also provide advantages in many other applications.
Conventional composite laminated structures historically exhibit static and fatigue characteristics that may permit a certain degree of micro-cracking of the structure to form and exist prior to ultimate failure of the structure. Such is due, at least in part, to the stress differential between first ply failure (FPF) and last ply failure (LPF), as commonly known and referred to in the art and as will be described in further detail below. In many applications such micro-cracking is tolerable, making conventional composite laminated structures suitable, at least in this regard. Certain applications, however, cannot tolerate micro-cracking, requiring alternatively designed structures that minimize the stress differential between FPF and LPF. Of course, with at least the previously described symmetry and balance constraints, conventional composite laminated structures with four or more ply angles are generally not suitable for such applications.
Accordingly, a need exists to provide laminate structures and methods of manufacturing and using the same, which minimize the various above-mentioned inefficiencies and limitations of balanced and symmetrical laminate structures, minimize micro-cracking, and expand the first ply failure envelope, all without sacrificing physical properties.